Microchip - Yb fiber hybrid optical amplifier for micro-machining and marking

ABSTRACT

The invention describes techniques for the control of the spatial as well as spectral beam quality of multi-mode fiber amplification of high peak power pulses as well as using such a configuration to replace the present diode-pumped, Neodynium based sources. Perfect spatial beam-quality can be ensured by exciting the fundamental mode in the multi-mode fibers with appropriate mode-matching optics and techniques. The loss of spatial beam-quality in the multi-mode fibers along the fiber length can be minimized by using multi-mode fibers with large cladding diameters. Near diffraction-limited coherent multi-mode amplifiers can be conveniently cladding pumped, allowing for the generation of high average power. Moreover, the polarization state in the multi-mode fiber amplifiers can be preserved by implementing multi-mode fibers with stress producing regions or elliptical fiber cores These lasers find application as a general replacement of Nd: based lasers, especially Nd:YAG lasers. Particularly utility is disclosed for applications in the marking, micro-machining and drilling areas.

This is a continuation of application Ser. No. 11/339,679 filed Jan. 26,2006, which is a continuation of U.S. application Ser. No. 11/141,704filed Jun. 1, 2005 and Ser. No. 10/645,662 filed Aug. 22, 2003, which isa continuation of Ser. No. 09/317,221 filed May 24, 1999, which is acontinuation in part of Ser. No. 09/116,241 filed Jul. 16, 1998, U.S.Pat. No. 6,208,458 which is a continuation in part of Ser. No.08/822,967, filed Mar. 21, 1997, U.S. Pat. No. 6,181,463. The entiredisclosures of the prior applications are considered part of thedisclosure of the accompanying continuation application and is herebyincorporated by reference.

FIELD OF THE INVENTION

This invention relates generally to laser systems having application tosuch fields as micro-machining, drilling and marking. A primarycharacteristic of these lasers is their high-powered short-pulsedoutput, which in, for example, an industrial application, preferablymachines the surface of a target or workpiece by an ablation technique.The invention also relates generally to laser systems which can serve inreplacement of more expensive Nd based lasers, such as diode-pumpedQ-switched Nd:YAG lasers and other lasers using Nd-based materials.

BACKGROUND OF THE INVENTION

It has been known in the prior art to use pulsed laser systems to effectsuch processes as diverse as metal machining and biological tissueremoval. Of chief concern in these systems is the amount of “collateraldamage” to the surrounding regions of the workpiece, or, in the case ofbiological uses, surrounding tissues. In the case of the machining ofmetallic workpieces, for example, laser pulses greater than 100microseconds in duration will machine the workpiece at the cost ofcreating a significant pool of molten liquid which is ejected from thebeam impact site. Cleanly machined features cannot be obtained with thismachining technique owing to the tendency of the molten material tospatter the workpiece and/or freeze and harden on the workpiece itself.This effect is due, of course, to the transfer of a significant amountof heat into the workpiece material at the target zone and atsurrounding areas as well. In the case of biological procedures, thisheat transfer effect typically causes unacceptable collateral damage tothe surrounding tissues.

A general but partial solution to this problem resides in the use ofshorter pulse durations. With shorter pulses the target is heated morequickly and thus reaches the evaporation point before significant liquidis permitted to form. Thus, in this arena, the shorter Q-switchedtemporal pulse may find advantage in certain applications. The pulsewidths of conventional Q-switched, solid state lasers used in micromachining is approximately 50-200 nanoseconds. This pulse width has formany cases proven to provide a reasonable balance between laser cost,machining accuracy and collateral effects such as the size of theheat-affected zone (HAZ), it being generally understood that the cost oflaser systems of significant power increases greatly with the shortnessof the period of the output pulse.

However, even in the above mentioned pulse width range, the degree ofheat transfer into the material is unacceptable for many applications.Recently developed lasers reported at OE/LASE SPIE vol. 2380 pp 138-143(1995) which generate pulses in the 8-20 ns range abate this problem toa degree, however since the threshold for ablation in the nanosecondrange decreases as the reciprocal of the square root of the lasertemporal pulse width, it is apparent that as the pulsewidth is furtherreduced, the range of potential applications broadens considerably.

With advances in pulsed laser systems, lasers having pulse widths wellinto the femtosecond regime have become available. At these ultrashortpulse widths, collateral damage to surrounding regions becomes almostnegligible, because of the lack of significant heat transfer into zonesoutside of the immediate target area. Essentially, the material at thetarget is substantially instantaneously vaporized while the fleetingduration of the impact of the laser energy substantially eliminates thepossibility of heat transfer into surrounding areas. In general, it isknown that the heat penetration depth L is proportional to the squareroot of the product of the heat diffusion coefficient (specific to thematerial) and the pulse width t. Consequently, as the pulse widthbecomes shorter, the heat penetration depth decreases proportionately.With femtosecond pulses, ablation thus takes place before significantheat can be transferred into the material, so that little or no heateffected zone (HAZ) is created. U.S. Pat. Nos. 5,656,186 and 5,720,894,incorporated herein by reference, discuss the above effects generally,and disclose laser systems operating well into the femtosecond regime insome instances.

However, as previously mentioned, the costs associated withfemtosecond-regime micro-machining lasers are not insignificant; theypresently cost five to fifteen times more than the presentnanosecond-regime micro-machining sources. Thus, there is a need in theindustrial and medical fields for a micro-machining or marking laserwhich reduces the collateral damage problems of the prior art, yet has acost comparable to the present sources. This goal has been achievedthrough the present invention, which, through the use of a novel andhighly efficient combination of Q-switching and Yb fiber lasertechniques, provides a source operating in the short nanosecond orsub-nanosecond regime which is less expensive than the micro-machiningsources now conventionally used, generating pulses as much as 4 ordersof magnitude smaller than that in the known micromachining arts, andthus producing a greatly decreased heat affected zone which is practicalfor a wide variety of applications while avoiding the greatly increasedcost of present femtosecond systems.

As mentioned above, Q-switching is currently a common technique forgenerating nanosecond optical pulses. It is known that the mainparameter which determines the duration of a Q-switched laser pulse isthe laser cavity round-trip time T_(round-trip)=2L_(cavity)/C, where cis the speed of light and L_(cavity) is the laser cavity length.Therefore, shorter laser cavity length is generally required forgenerating shorter Q-switched pulses. However, it is known that thisshortening of the cavity length normally reduces the mode volume whichmakes if more difficult to achieve suitable pulse energies. Furtheramplification in a solid-state amplifier is usually not a practicalsolution due to the very low gain characteristic of solid-stateamplifiers. Moreover, pushing the energies from a short pulse microchiplaser sufficient for micromachining, reduces the microchip laserefficiencies to around 5%.

Here we demonstrate that by using a low energy microchip laser inconjunction with a highly efficient large core Yb fiber amplifier theseproblems can be overcome and subnanosecond optical pulses can beachieved at high pulse energies.

Known Nd: based lasers, in addition to being expensive, are lessefficient compared to Yb-doped fiber amplifiers. For example, Nd:YAGlasers transform the diode pump power to optical output at approximately50% efficiency. In contrast, Yb fiber amplifiers transform laser diodepump power to optical output with about 90% efficiency. This betterefficiency leads to certain cost savings, especially when the comparisonis based on cost per unit of output power.

The amplification of high peak-power and high-energy pulses in adiffraction-limited optical beam in single-mode (SM) optical fiberamplifiers is generally limited by the small fiber core size that needsto be employed to ensure SM operation of the fiber. To overcome theenergy and peak power limitations, recently the use of multi-mode (MM)fiber amplifiers has been suggested (U.S. Pat. No. 5,818,630 to Fermannand Harter, herein incorporated by reference). In this work the loss ofspatial beam quality in MM fiber amplifiers is prevented by excitationof the fundamental mode via the use of appropriate mode-matching bulkoptics or fiber tapers as suggested in U.S. Ser. No. 09/199,728 toFermann et al., herein incorporated by reference.

Particularly interesting are MM fiber amplifiers that are double-cladsince they can be conveniently pumped with high-power diode lasers toproduce high average powers. Moreover, the achievable smallcladding/core ratio in double-clad MM fibers also allows the efficientoperation of fiber lasers with small absorption cross sections, assuggested in the aforementioned U.S. Pat. No. 5,818,630 to Fermann andHarter.

Cladding-pumped fiber amplifiers and lasers have been known for manyyears. See U.S. Pat. No. 4,829,529 to J. D. Kafka, U.S. Pat. No.4,815,079 to Snitzer et al., U.S. Pat. No. 5,854,865 to Goldberg, U.S.Pat. No. 5,864,644 to DiGiovanni et al., and U.S. Pat. No. 5,867,305 toWaarts et al. In the early work in this area (Kafka and Snitzer) onlydouble-clad fiber amplifiers comprising a SM core were considered forcladding-pumping, resulting in obvious limitations for the amplificationof high peak power pulses. Moreover, Snitzer et al. only considereddouble clad fibers with approximately rectangular-shaped ornon-centrosymmetric cladding cross sections to optimize the absorptionefficiency of such fibers. The use of relatively small cladding/corearea ratios enabled by double-clad fibers with a large multi-mode core,however, allows for the efficient implementation of any arbitrarycladding cross section, i.e. circular, circular with an offset core,rectangular, hexagonal, gear-shaped, octagonal etc. The work by Kafkawas equally restrictive in that it only considered double-clad fiberswith a single-mode core pumped with coherent pump diode lasers. Againthe use of relatively small cladding/core area ratios enabled bydouble-clad fibers with a large multi-mode core enables the efficientimplementation of pump diode lasers with any degree of coherence.

The later work of Goldberg and DiGiovanni was not necessarily restrictedto the use of double-clad fibers with SM fiber cores. However, none ofthe work by Goldberg and DiGiovanni (or Kafka, Snitzer or Waarts et al.)considered any technique for the effective use of multi-mode double-cladfibers as diffraction-limited or near diffraction-limited high-poweramplifiers. No methods were described for exciting the fundamental modein multi-mode amplifiers, no methods were described for minimizingmode-coupling in multi-mode amplifiers and no methods were described forcontrolling the excitation and the size of the fundamental mode bygain-guiding or by the implementation of an optimized distribution ofthe dopant ions inside the multi-mode fiber core.

Moreover, the specific pump injection technique suggested by DiGiovannicomprises built-in limitations for the efficiency of fundamental-modeexcitation in multi-mode fiber amplifiers. DiGiovanni considers a fusedtaper bundle with a single-mode fiber pig-tail in the center of thebundle, which is then spliced to the double-clad amplifier fiber tosimultaneously deliver both the pump light (via the outside fibers ofthe fused taper bundle) and the signal light (via the single-mode fiberpig-tail) to the amplifier fiber. Due to the limited packing ability ofcircular structures, air gaps remain in the fiber bundle beforetapering. Once tapered, surface tension pulls all the fibers in thefiber bundle together, essentially eliminating the air gaps (asdiscussed by DiGiovanni et al.). As a result the outside cladding of thetaper bundle becomes distorted (resulting in a non-circular shape withridges where the fibers were touching and with valleys where there wereair-gaps). Hence the central core region and the fundamental mode alsobecome distorted which limits the excitation efficiency of thefundamental mode in a MM fiber when splicing the fiber bundle to thedouble-clad fiber. In fact any geometric differences in the claddingshape of the fiber bundle or the double-clad fiber will lead to alimited excitation efficiency of the fundamental mode in the MM fiber inthe process of splicing.

For reducing size and cost of the system as well as for increasingefficiency of the amplification side-pumping (as described inaforementioned U.S. Pat. No. 5,818,630) rather than end-pumping might beadvantageous. For the benefits of fiber reliability the use of fibercouplers is preferred. The use of fiber couplers for pump lightinjection into MM fibers is discussed in aforementioned U.S. Ser. No.09/199,728.

Normally for many applications a single polarization is desirable, sothe use of polarization preserving fiber is desirable. There are severalmeans of making polarization preserving fiber. However, for multimodefiber, elliptical core fiber is the easiest to manufacture and to obtainat this time.

Another attractive feature would be ease of fiber coupling the laser tothe application, by using the amplifier fiber as the fiber deliverysystem, or a multimode undoped fiber spliced to the end of the amplifierfiber. This is similar to the fiber delivery system described in U.S.Pat. No. 5,867,304 and its progeny, herein incorporated by reference,where a multimode fiber is used for delivery of a single mode beam. Thepurpose is to lower the intensity in the fiber by using the largereffective mode-field diameter. This allows higher peak powers; >1 KWpulses can be transmitted without the onset of nonlinear processes. InU.S. Pat. No. 5,867,304, this fiber is used with ultrashort pulses wherethe fiber dispersion distorts the pulses. However, with nanosecondpulses, dispersion has a negligible effect on the pulse width sodispersion compensation is not necessary.

SUMMARY OF THE INVENTION

According to the invention, the goals set out in the foregoing areachieved through the use of a miniature Q-switched pulse source which iscoupled to a doped Yb fiber laser which obtains single modeamplification in a multi mode fiber. Short pulse duration, efficiency,high power, high energy, cost efficiency and compactness are essentiallyachieved through the use of the combination of a compact diode-pumpedmicrochip laser and a specially designed diode-pumped fiber amplifier.Short duration is achieved through the short cavity length of amicrochip laser, whereas high efficiency is achieved through the use ofa Yb-doped fiber amplifier pumped at 980 nm. High power is achievedthrough cladding pumping geometry, and large fiber core (high core tocladding ratio).

High energy is achieved through a number of design features: the largecore, with single mode excitation and propagation, allows a largecross-sectional area and, consequently, permits relatively low peakintensities and high saturation energies. Further, the large coreprovides a good core-to-cladding ratio, which in conjunction with thehigh doping level available for Yb significantly reduces the pumpabsorption length and allows for short amplifier lengths (0.1 to ˜2 m),thus reducing detrimental nonlinear effects in the fiber withoutcompromising power and energy extraction efficiencies. For very largecores, direct in-core pumping can be used. Side pumping provides higherpower extraction efficiency and shorter interaction length compared tocopropagating geometries (along with pump diode protection). Pigtailingof the fiber ends increases the surface damage threshold and allows asignificant increase in output pulse energies and powers, while acomposite core allows the robust coupling of the microchip seed pumpinto a fundamental mode of the fiber core. This also permits use of anon-perfectly-gaussian input beam from the microchip laser.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic layout of the laser system of the invention;

FIG. 2 illustrates schematically one actively Q-switched micro-laseraccording to the invention;

FIG. 2 a illustrates a typical layout of the actively Q-switched microchip laser;

FIG. 3 illustrates the temporal profile of the output of the lasers ofFIGS. 2 and 2 a; and

FIGS. 4 and 4 a, where FIG. 4 is presented as an inset in FIG. 1,illustrate a fiber-end coupling and optical damage avoidance technique.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the system configuration of the laser according tothe present invention. In this Figure, reference numeral 101 indicates amicrochip laser source, illustrated in greater detail in FIGS. 2 and2(a). It should be noted that, as used herein, the term “micro chiplaser” refers to a laser of small device size, where at least some ofthe components, such as the gain medium and the end mirror, aremonolithic. In this specification, the terms “microchip laser” and“microlaser” are used interchangeably to refer to a laser having thesecharacteristics. As described in detail below, the micro chip laser 101according to the invention is an actively Q-switched laser which istypically diode pumped.

In order to achieve excitation of only the fundamental mode in amultimode-core fiber amplifier, the beam waist ω_(input) of a modecoupled into the amplifier from a microchip laser has to approximatelymatch the beam waist ω_(mode) of this fundamental mode:ω_(input)≈ω_(mode). Note, that for the step-index fiber ω_(mode)=0.7r_(core), where r_(core) is the radius of a fiber core. Therefore, theoutput of the microchip laser 101 has to be directed into the fiberamplifier input (FIG. 4) through properly designed mode-matching optics102. The essential function of this mode-matching optical arrangement isto transform the mode size of an optical beam at the output of amicrochip laser ω_(output) into the proper beam size ω_(input) at theinput of the fiber amplifier. This imaging function can be achieved by avariety of optical arrangements, one example of which is schematicallyrepresented in FIG. 1. Note that the focusing lens in this arrangementis also used to focus the pump light from a laser diode, and that it isessential for our invention to achieve focusing of these two input beamsat two different planes, as described below.

The inventors have determined experimentally that limitations on themaximum extractable energies in a fiber amplifier originate from anumber of effects, two significant ones being the Raman gain and surfacedamage at the input and output facets of the fiber core.

The optical damage threshold at the surface of a glass is characterizedby the optical intensity I_(th) ^(damage) of an optical beam at thissurface. Generally, this threshold intensity is determined by the typeof material used and by its surface quality. It also depends on theduration of the pulse and average power (repetition rate) of the pulsetrain. As is known, the threshold intensity for optical damage in thenanosecond range decreases as the reciprocal of the square root of thelaser temporal pulsewidth: I_(th) ^(damage)∝1/√T_(pulse).

The inventors have demonstrated that the optical surface damagethreshold can be significantly increased by using a beam expansiontechnique, as shown schematically in FIG. 4 and in greater detail inFIG. 4 a. Here, the fiber-end is bonded to a buffer of the same materialas the fiber. At the end surface, the optical beam will be expanded toω_(expanded) according to:$\omega_{expanded} = {\omega_{mode}\sqrt{\left( {1 + {2{L/\omega_{mode}^{2}}k}} \right)}}$

Here, k=2πn/λ, n is the glass refractive index, λ is the wavelength ofthe amplified signal and L is the thickness of the buffer. It iscritical that the quality of the bond between the surfaces of the fiberand the buffer be sufficiently high to eliminate any optical interface,and, thus, to eliminate surface damage at this surface. Various knownbonding techniques can be used to achieve this quality. In the presentcase, a silica-glass rod of the same diameter as the outer diameter ofthe pump-cladding was spliced to the end of the fiber. The maximumimprovement η of the damage threshold is determined by the square of theratio between the radius of the buffer rod R_(buffer) and the size ofthe core mode ω_(mode):η=(R_(buffer)/ω_(mode))². In the case of a 50micron core and a 300 micron buffer pigtail as used in our experimentalconfiguration the improvement was found to be ˜70 times. Suchbuffer-pigtail protection is required for both input and output ends ofan amplifier. In the case signal and pump beams are entering the sameend of a fiber (copropagating configuration) the incoming laser beam hasto be focused on the end of the fiber, as shown in FIG. 4 a, inside thebonded buffer, where there is no interface. If the bonded buffer is acoreless rod of the same diameter as fiber-amplifier inner cladding(pump cladding), as shown in FIG. 4 a, the pump beam should be focusedat the entrance facet of this silica rod. Note, that generally thisbuffer can be a slab with transverse dimension much larger than the pumpcladding. In this case pump beam could be directly focused into the pumpcladding. In the case side pumping is used via a V-groove or a fiberpigtail the corresponding element can be either placed directly in thefiber amplifier after the buffer bonding point, or (if a silica rod isused as a buffer) in this coreless pigtail.

The Raman effect causes the spectrum of the amplified pulse to shifttowards the longer wavelengths and outside the amplification bandwidthof the Yb-fiber amplifier. Raman effect onset is characterized by athreshold intensity I_(th) ^(Raman) in the fiber core which, as is knownin the prior art, is inversely proportional to the effective propagationlength L_(eff) of an amplified pulse and the Raman gain coefficient:I_(th) ^(Raman)∝1/L_(eff) g_(Raman). Since the Raman gain coefficient isdetermined by the fiber glass properties, in order to maximizeextractable peak powers and, hence, pulse energies, one has to increasethe core size and decrease the interaction length. The interactionlength can be reduced by using fibers with high doping level whichlowers the fiber length, propagating amplified pulses opposite to thedirection of the pump beam which lowers the pulse energy until the endof the fiber where the gain will be higher. Also, use of multimode largecore fibers in the double clad configuration facilitates pump absorptionand allows shorter amplifier lengths.

It is important to note that for certain applications the presence ofstrong Raman components in the amplified pulses does not reduce theusability of these pulses. One example is laser marking. The inventorsdemonstrated experimentally that surface marking is not sensitive to theRaman spectral shift and there is no degradation in the marking qualityeven for pulses with only a small fraction of the total energy innon-Raman shifted spectral components. In one specific example, thisallowed use of 150 μJ of total pulse energy vs ˜40 μJ that was availablewithout Raman shifting. Thus, for this type of application significantlyhigher energies are available from this particular fiber amplifier.

However, many applications are sensitive to the presence of the Ramanshift. For example, when wavelength shift is required prior to end use,via second-harmonic or other frequency conversion methods, the Ramancomponent would significantly reduce the efficiency of this conversionand would produce large amplitude fluctuations. For such applications, anumber of existing techniques currently employed in fibertelecommunication systems (See, OFC'95 Tutorial Session) could be usedfor Raman-effect reduction in the fiber amplifiers, in addition to themethods described in this invention for optimizing fiber amplifiers inorder to minimize their susceptibility to Raman effect.

The fiber amplifier 103 is a Yb-doped large-core cladding-pumped fiberamplifier. The core diameter of this fiber is approximately 10micrometers-1 mm in diameter and thus is a true multimode fiber.However, this multimode fiber performs single mode amplification usingthe techniques described in U.S. Pat. No. 5,818,630, herewithincorporated by reference.

Reference numeral 104 illustrates the pump for the Yb multimode fiberlaser. The pump is advantageously configured as a side-pumping broadarea laser diode, the details of which are well known in the art. The Ybfiber amplifier can transform the pump power into an optical output withan extremely high efficiency of 90%. In addition, the multimode Ybamplifier fiber produces an output which is higher by more than an orderof magnitude over that obtainable with a corresponding conventionalsingle mode fiber amplifier. The combination of extremely highefficiency and high gain allows the source microchip laser to operate ina relatively low energy, higher efficiency regime with little inputpower.

FIGS. 2 and 2(a) illustrate two preferred embodiments of the micro-laseror microchip laser used according to the invention. These devices areextremely compact, simple, inexpensive and have low power requirements,yet produce extremely short high peak power pulses. According to theinvention, the microlasers employed are diode pumped lasers which areactively Q-switched. A primary advantage of these miniature lasers isthat they readily provide output laser pulses of very short duration asa consequence of their short laser cavities. Active Q-switching givesgood control over the repetition rate and the number of pulses deliveredat a time, which is useful in marking and micromachining applications.

The microchip laser is a solid-state device designed to providenanosecond laser pulses at 1064 nm wavelength. Diode pumping enableshigh pump-to-laser efficiency, compact design, and reduced thermalproblems in the gain material. The cavity is designed to provide theshortest possible pulse duration achievable with active Q-switching withmoderate (3 micro J) pulse energy.

Two representative laser cavity designs are shown in the Figures. Thegain material is Nd doped Yttrium Orthovanadate (Nd:YVO₄) at 1% dopinglevel. It is cut and oriented in a way (a-cut) to provide maximumabsorption at the pump wavelength. In addition, the crystal is wedgeshaped in FIG. 2 a, which allows the laser to operate only in one linearpolarization. The crystal is pumped longitudinally through its coateddichroic dielectric mirror surface 201. The pump laser 203 is a 100micron wide laser diode with 1 Watt cw pump power. The coating 201provides passage of pump light at 808 nm and reflection of laser lightat 1064 nm. This surface acts also as a laser cavity mirror. The laserhas a flat output coupler. Some thermal focusing in the cavity tends tostabilize the laser cavity mode, but it is basically an unstableresonator.

A Pockels cell 207 and a quarter-wave plate 209 inside the cavity forman electro-optic Q-switch. The Pockels cell is made of LiNbO₃, in thetransversal field configuration. The Pockels cell at the off state haszero retardation. The quarter-wave plate provides a static half waveretardation of light in a round trip, which means changing thepolarization of light inside the cavity. This opposite polarization isthen deflected out of the cavity (FIG. 2 a) by the wedge shaped gainmaterial acting as a polarizer, or a polarizer is placed inside thecavity (FIG. 2). The laser is in the static off state with the voltageoff at the Pockels cell. When the gain material is pumped continuously,the pump energy is stored in the gain material for approximately 100microseconds, the fluorescence lifetime of the gain material. ToQ-switch the laser, a fast, 2.5 ns rise time high voltage pulse (1200 V)is applied to the Pockels cell. The voltage on the Pockels cellintroduces a quarter-wave retardation, which compensates the retardationof the wave plate. The intra-cavity laser field then builds up unimpededuntil it finally reaches saturation by depleting the gain. The laserpulse leaves the cavity through the output coupler 211, which has 70%reflectivity and 30% transmission. The resulting laser pulse has 750 pspulse duration and 3 micro J energy (FIG. 3). A solid-state drivingelectronics circuit provides the fast, high voltage switching pulses forthe Pockels cell with a repetition rate up to 15 kHz. To operate thelaser as a cw source a static voltage can be applied to the Pockelscell.

Single longitudinal mode operation is often desired in lasers. Besidesthe favorable spectral properties to the laser, single-mode operationreduces the timing jitter. In single longitudinal mode operation thereis no mode competition and gain cross-saturation between modes. As aresult, the uncertainty of the turn-on time of the laser relative to thetrigger pulse, the jitter, is reduced. Timing jitter of less than 100 psis obtained when the laser operates in single mode.

The laser cavity is designed for single-longitudinal-mode operation. Forlong term stability it is particularly important that the laser cavityis stabilized against temperature induced changes. The cavity isdesigned so that temperature induced effects do not cause mode-hoppingin the laser. The mechanical and optical construction of the laser issuch that the thermal expansion of the base whereon the laser in mountedcompensates for the thermal effects in the materials. In addition tothermal expansion, further consideration was given to high thermalconductivity and good electrical and mechanical properties of the basematerial, which enables temperature stabilization of the components.

Because the length of the resonator is approximately 8 mm, the laser cansupport 4 to 6 longitudinal modes at this cavity length. To achievesingle mode operation we employed a resonant reflector etalon outputcoupler. The use of an resonant reflector etalon to maintain single modeoperation is described in Koechner pp. 242-244. The output coupler is asolid Fabry-Perot etalon working in the reflection mode. Itsreflectivity R is modulated as a function of wavelength. The maximumvalue of reflectivity occurs at the resonant wavelengths given byδ_(etalon)/2π=m,  (1)where δ_(etalon) the phase difference between interfering optical beamsin the etalon at consecutive reflections m is a half integer number(m=½, 3/2, 5/2, . . . ).On the other hand, resonant wavelengths of the laser cavity aredetermined by the total optical phase difference between beams ofconsecutive reflections inside the cavity, δ_(cav),δ_(cav)=4πΣ(n _(i) l _(i))/λThe summation takes into account all the optical materials; gainmaterial, Pockels cell, polarizer and quarter-waveplate material and airwith their respective optical thickness n_(i) l_(i). The resonantcondition for the cavity isδ_(cav)/2π=n,  (2)where n is an integer value (n=1, 2, 3, . . . ). Lasing occursessentially when the resonant wavelength of the output coupler etaloncoincides with the resonant wavelength of the laser resonator cavity.This is given by simultaneous satisfaction of the above half-integer andinteger conditions for m and n respectively. The number of allowablemodes under the gain profile can be restricted to 1 by proper choice ofthe output coupler etalon. In our embodiment of the microlaser a singleuncoated LiNbO₃ plate of 1 mm thickness provides sufficient modeselectivity to allow the laser to operate in a single longitudinal mode.The resonance conditions (1) and (2) are temperature dependent, sincethe thermal expansion and the thermal change of the refractive indexchanges the optical path-length in the laser cavity and in the resonantreflector output coupler. These effects combine to shift the resonancepeaks of the resonant reflector and the laser cavity. We have a limitedchoice of the optical materials from which the laser is constructed.Their thermal expansion constants and thermal induced refractive indexcoefficients determine the thermal change of resonance conditions, whichin general results in a mismatch of resonances (1) and (2) as thetemperature changes and causes mode hopping of the laser. The thermalexpansion of the base on which the laser is constructed also contributesto the change of the wavelength of the laser. We have a rather freechoice of the base material. By using Aluminum Nitride ceramic as thelaser base we achieved that the thermal shift of the laser wavelengthwas matched to the thermal shift of the resonance condition of theresonant reflector output coupler and mode hopping has not occurredwithin a 4 degree C. temperature interval. Temperature stabilization ofthe laser cavity within 1 degree C. resulted in continuous singlelongitudinal mode operation of the laser.

An alternative source may be a passively Q-switched microchip laser,which can be very inexpensive and may be preferred in some cases forthis reason. The primary reason to use a miniature source is to keep thelaser cavity short which reduces the pulse width of the laser.

The miniature laser is coupled to a doped fiber gain medium. In theinvention this medium is a Yb:fiber.

In order to reach higher peak powers, the invention utilizes amulti-mode fiber to propagate single mode pulses as described in U.S.Pat. No. 5,818,630. As described above a mode converter is used toconvert the single mode input to excite the fundamental mode of themultimode fiber. The mode converter 102 used in this case is acombination of lenses which mode-matches the output of the microchiplaser to the beam diameter for single mode excitation of the multimodefiber. In addition to the lenses for mode-conversion, gain guiding inthe Yb:fiber can be used to relax the tolerances on mode matching.Without gain in the Yb fiber, robust fundamental-mode excitation becomesincreasingly difficult to achieve for the increasing core size of afiber amplifier. We found experimentally that it is particularlyadvantageous to employ specially designed fibers in which Yb-doping inthe center of the core has a significantly smaller diameter than thecore itself. In this case, the fundamental mode light experiencessignificantly higher gain than multimode light. In our experimentalconfiguration, we used 50 μm diameter core with 25 μm diameter dopedregion in the center, which exhibited a significantly more robustperformance compared to 25 μm homogeneously doped core. Besides relaxingthe alignment tolerances, the beam parameters of the source are alsorelaxed. As the microchip laser may not have a perfect diffractionlimited beam output, gain guiding can be used to correct for this. Also,gain guiding can correct the distortion expected from DiGiovanni pumpcouplers.

The Yb fiber in this example had a 300 μm outer diameter and a 50 μmcore. The use of relatively small cladding/core area ratios enabled bydouble-clad fibers, together with a large multi-mode core, allows forthe efficient absorption of the pump with, for example, a gear-shapecladding cross section. The resultant Yb amplifier can be as short as1.5 M long, as compared to 5-40 M which would be required of a typicalsingle mode Yb amplifier.

Another advantage of this optical source is the ease of adding amultimode fiber delivery system which propagates a single-mode. In manyapplications fiber delivery is very important, such as in surgery,dentistry and marking in confined spaces. An example of marking inconfined spaces is the marking of assembled automotive or other partsfor antitheft purposes.

An additional advantage of the shorter pulse is that nonlinear processesfor frequency conversion are more efficient with the higher peak powerswhich come from shorter pulses with similar energies. For certainapplications where wavelength conversion is necessary, for example inUV-range radiation for via hole drilling, the output of the laser mustbe frequency tripled to create the UV radiation. This source, could, forexample, replace frequency tripled Q-switched Nd:YAG lasers and eximerlasers for this application.

Another application where frequency conversion is important isdentistry. For example, in U.S. Pat. No. 5,720,894, it is described thatUV radiation performs relatively damage free material removal by hardtissue ablation primarily due to the stronger absorption of thatwavelength regime. Three preferred wavelengths for applications inmedicine and dentistry are 2.1 μm, 2.9 μm and 1.55 μm. Like UVradiation, the preference is due to the strong absorption coefficient ofbiological tissues at these wavelengths.

The most straight forward means for generating 1.55 μm radiation is touse a laser source which emits at 1.55 μm and a doped fiber whichamplifies 1.55 μm radiation. A microchip laser which emits 1.55 μmradiation is known, and described in Thony et al. It is well known thaterbium fiber amplifies 1.5 μm radiation. An alternative source could bea compact erbium doped waveguide laser as described in; H. Suche, T.Oesselke, J. Pandavenes, R. Ricken, K. Rochhausen, W. Sohler, S.Balsamo, I. Montrosset, and K.K. Wong “Efficient Q-switched Ti;Er:LiNbO₃ waveguide laser”, Electron. Lett., Vol. 34, No. 12, 11 Jun.1998, pp 1228-1230.

Another alternative is to use a laser source which emits a differentwavelength, such as that of the invention, and use a frequencyconversion step to generate the 1.5 μm radiation. Examples of anonlinear conversion step at the output include doubling, tripling,quadrupling, Raman shift, OPO, OPA or OPG. To generate 1.55 μmradiation, converting a 1.06 source in a PPLN OPG is quite convenient.

In order to generate other wavelengths such as 2.1 and 2.9 μm similarmethods can be applied to this laser concept.

The multimode amplifier of the invention can also amplify a cw source oroperate as a cw source. For example, a marking laser often has theoption of being operated in a cw mode for generating more of a heat typemark. For the design of high-power cw lasers the use of MM fibers isadvantageous as the reduced cladding/core area ratio reduces theabsorption length in such structures. For very high cw laser powers,nonlinear effects can indeed occur and thus MM fibers can be used forthe construction of compact ultra-high power cw fiber lasers. The MMfibers can then be effectively used for the pumping of fiber Ramanamplifiers or for the construction of Raman lasers operating atwavelength regions shifted away from the gain band of the doped fibers.

As previously indicated, a number of major advantages are achievedaccording to the invention by employing the combination of a Q-switchedmicrochip laser and a Yb: fiber amplifier. Because of the efficiency andgain of the Yb fiber amplifier, the output power of the microchip laserneed not be large. The peak power of this amplifier is limited bynonlinear effects in the fiber and by the optical damage thresholdsprimarily at the fiber ends. The delivery fiber may be a simplemultimode undoped fiber spliced to the end of the amplifier fiber, orthe amplifier 103 can itself constitute the fiber delivery system. Thus,a simple, inexpensive laser system suitable for a wide variety ofapplications can be efficiently produced.

1. A buffer pigtail, comprising; a coreless element bonded to at leastone of input and output ends of a fiber amplifier substantially withoutan interface therebetween, and having a composition matching that of thefiber of said fiber amplifier; said pigtail functioning to increase anoptical damage threshold of the fiber amplifier.
 2. A buffer pigtail,comprising; a light input end; a light output end coupled to a fiberamplifier in a manner as to eliminate or minimize any interfacetherebetween; said pigtail having a composition matching that of thefiber of said fiber amplifier, and functioning to increase an opticaldamage threshold of said fiber amplifier.
 3. A buffer pigtail,comprising; a light output end; a light input end coupled to a fiberamplifier in a manner as to eliminate or minimize any interfacetherebetween; said pigtail having a composition matching that of thefiber of said fiber amplifier, and functioning to increase an opticaldamage threshold of said fiber amplifier.
 4. A fiber amplifiercomprising; a transverse cross section that comprises at least one coreregion and one cladding region, said amplifier further containing atleast one end face used for the extraction of optical energy from saidfiber amplifier, said optical energy being contained in a beam of acertain diameter; said end face further having a transverse crosssection which differs from the transverse cross section at fiberlocations remote from said end face, the diameter of said extractedoptical beam at said at least one fiber end face being larger than thediameter of said fiber core.
 5. A fiber amplifier comprising; at leastone core region and one cladding region, said amplifier furthercontaining at least one end face used for the extraction of opticalenergy from said fiber amplifier, said optical energy being contained ina beam of a certain diameter; the diameter of said extracted opticalbeam at said at least one fiber end face being larger than the diameterof said fiber core; said increase in diameter at said fiber end facefunctioning to increase an optical damage threshold of said fiberamplifier.